The lithium secondary battery has recently been widely used, for example, as an electric source for driving small-sized electronic devices. The lithium secondary battery comprises a positive electrode, a negative electrode and a non-aqueous electrolytic solution. The positive electrode generally comprises complex oxide of lithium such as LiCoO2, and the negative electrode generally comprises a carbon material or metallic lithium. A cyclic carbonate such as ethylene carbonate (EC) and/or a linear carbonate such as dimethyl carbonate (DMC) have generally been used as a non-aqueous solvent in the non-aqueous electrolytic solution for the lithium secondary battery.
The recent secondary battery should give a high voltage and a high energy density. It is difficult, however to improve both the battery performances and the safety. A battery of a high energy density should particularly show high protection from overcharge, compared with the conventional battery. It is also difficult to maintain the cycle characteristics and the storage stability at high temperatures. Further, the battery tends to generate a gas, which may expand the battery. In consideration of the recent requirements on the lithium secondary battery, the performances of the battery so far developed do not satisfy the requirements. Therefore, the secondary battery should be further improved in safety while keeping the battery performances to satisfy future requirements for the high energy density.
Japanese Patent Provisional Publication No. 2003-317803 discloses an invention of adding sec-alkylbenzene or cycloalkylbenzene in which a fluorine atom is attached to the benzene ring to a non-aqueous solvent of a non-aqueous electrolytic solution for a lithium secondary battery. The publication reports that a lithium secondary battery of a high energy density shows an excellent safety performance in a function of terminating progress of overcharge by using the solution without lowering battery performance. The publication further describes that the non-aqueous solvent can further contain various known non-aqueous solvents. In working examples of the publication, ethylene carbonate (cyclic carbonate) and diethyl carbonate (linear carbonate) are used in a weight ratio of 1:1 with a small amount of vinylene carbonate.